1. Field of the Invention
The present invention relates to a membrane electrode assembly for use in a solid polymer electrolyte fuel cell.
2. Description of the Related Art
Oil resources have been depleted, and at the same time, environmental problems including the global warming caused by fossil fuel consumption have been increasingly serious. Accordingly, fuel cells have attracted attention as clean electric power supplies for electric motors not involving the generation of carbon dioxide, and thus have been extensively developed and partially begin to be used practically. When the fuel cells are mounted in automobiles and the like, solid polymer electrolyte fuel cells using solid polymer electrolyte membranes are preferably used because such fuel cells can easily provide high voltage and large electric current.
Known as a membrane electrode assembly to be used in the solid polymer electrolyte fuel cell is a membrane electrode assembly which comprises a pair of electrode catalyst layers disposed on both sides of a solid polymer electrolyte membrane having proton conductivity, and gas diffusion layers laminated respectively on the electrode catalyst layers. Each of the pair of the electrode catalyst layers is formed by supporting a catalyst such as platinum on a catalyst carrier such as carbon black and by integrating the supported catalyst into a single piece with an ion conducting polymer binder; one of the electrode catalyst layer acts as a cathode electrode catalyst layer and the other as an anode electrode catalyst layer. The gas diffusion layers are formed of, for example, carbon paper. The membrane electrode assembly constitutes the solid polymer electrolyte fuel cell in combination with separators each doubling as a gas path and respectively being laminated on the gas diffusion layers.
In the solid polymer electrolyte fuel cell, the anode electrode catalyst layer is used as a fuel electrode into which a reductive gas such as hydrogen or methanol is introduced through the intermediary of the gas diffusion layer, and the cathode electrode catalyst layers is used as an oxygen electrode into which an oxidative gas such as air or oxygen is introduced through the intermediary of the gas diffusion layer. In this configuration, protons and electrons are generated in the anode electrode catalyst layer from the reductive gas by the action of the catalyst contained in the electrode catalyst layer, and the protons migrate to the electrode catalyst layer of the oxygen electrode side through the solid polymer electrolyte membrane. The protons react with the oxidative gas and the electrons introduced into the oxygen electrode to generate water in the cathode electrode catalyst layer by the action of the catalyst contained in the electrode catalyst layer. Consequently, connection of the anode electrode catalyst layer and the cathode electrode catalyst layer with a conductive wire makes it possible to form a circuit to transport the electrons generated in the anode electrode catalyst layer to the cathode electrode catalyst layer and to take out electric current.
As described above, in the membrane electrode assembly, the electric power generation is accompanied by the generation of water in the cathode electrode catalyst layer. Consequently, a long-time operation of the fuel cell makes excessive the moisture in the membrane electrode assembly to inhibit the diffusion of the reductive gas or the oxidative gas, and hence this case also suffers from a problem that no sufficient electric power generation performance can be attained. With respect to this problem, well known is a membrane electrode assembly in which the electrode catalyst layer is made water-repellent to facilitate the discharge of the generated water (see Japanese Patent Laid-Open No. 6-52871).
In the membrane electrode assembly, the protons migrate along with water in the solid polymer electrolyte membrane. Accordingly, the solid polymer electrolyte membrane needs to have appropriate moisture. Such moisture is supplied, for example, by the reductive gas or the oxidative gas. However, there is a problem in that no sufficient electric power generation performance can be attained under the low-humidity conditions that the humidity of the reductive gas or the oxidative gas is lower as compared to the humidity in the steady operation such as a high load operation.
A possible means for solving the above described problem is a regulation of the hydrophilicities in the cathode side and the anode side in the membrane electrode assembly. As a technique to regulate such hydrophilicities, for example, known is a membrane electrode assembly in which the gas diffusion layer in the cathode side is formed with a carbon-containing material and the surface of the carbon-containing material is modified to be hydrophilic (see Japanese Patent Laid-Open No. 2004-31325).
Also known is a membrane electrode assembly in which the catalyst carrier of the electrode catalyst layer is made hydrophilic (see Japanese Patent Laid-Open No. 6-275282).
However, the above described conventional techniques involve disadvantages, namely, fears that under the conditions that the temperature of the membrane electrode assembly is not sufficiently raised yet, for example, at the time of start-up and hence the humidity is higher than that in the steady operation, the water generated in the cathode electrode catalyst layer stays inside the membrane electrode assembly to inhibit the gas diffusion, and consequently no sufficient electric power generation performance can be attained.